Houses not decorated usually get left alone, or are less likely to be knocked on. You could also just not answer though, I suppose, if people don't pay attention to the lack of decor. I wouldn't answer if I hadn't decorated personally.
We don't do Hallowe'en ourselves, but is that ubiquitously understood that people who might want to decorate their houses for it are thereby also committing themselves to obliging trick or treaters?
A lot of people decorate their homes for Christmas but most of them wouldn't want to be disturbed by carol singers knocking on their door, raising money (even if for charity).
Some neighbourhoods near us have a popular Santa sleigh organised by the local Round Table, but the children have to go out to him when they hear him outside; they don't go around knocking on doors.
I also don't think it's quite as simple as 'no decorating = no trick or treaters', as a lot of children - usually the chancing teenagers - will knock on every door. They aren't doing it for innocent seasonal fun, but because they see it as a time of the year when they are entitled to go out and demand free stuff/sweets/money.
We used to live in a very rough area and we dreaded this every year; I imagine many elderly people would have been absolutely terrified. Please don't be under the illusion that everybody giving 'treats' is doing it with a happy smile; a lot see it as forcibly complying with a protection racket.
They didn't bother dressing up and, if you didn't give them something to 'make it worth their while' - or if you didn't answer the door at all, even if you were out or gone to bed - you would find there had been a 'trick' in the form of something unpleasant and persistent to have to scrub off your door or window the next day.